1. Field of the Invention
This invention relates to liquid crystalline compounds and mixtures.
2. Description of the Art
Liquid crystals have recently gained considerable importance primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, deformation of aligned phases (DAP type), the Shadt-Helfrich effect (rotation cell), the guest/host effect or a cholesteric-nematic-type phase transition.
Liquid crystals must satisfy a number of requirements in order to be suitable as dielectrics for electro-optical indicating devices. For example, they must have a good chemical stability towards environmental factors such as, for example, heat, air, moisture and the like, must be colourless, must have short response times and not too high a viscosity, must have a nematic or cholesteric mesophase in all temperature ranges in which the liquid crystal cell is to be operated and must give a good contrast. Other properties such as, for example, the threshold potential, the dielectric anisotropy and the electrical conductivity, must fulfil different conditions depending on the type of cell which is used.
Since, in general, it is not possible to achieve all desired and to some extent contradictory properties with a single compound, attempts have mainly been made to optimize the properties for the particular applications by mixing several compounds or components. In this case it is, however, important that the components undergo no chemical reactions with one another and can be mixed well. Further, the mixtures formed should have no smectic mesophases.
There are already known a series of liquid crystalline compounds which have as wing or terminal groups, for example, alkyl, alkoxy, alkanoyloxy, alkoxycarbonyl and cyano groups. However, a considerable disadvantage of the known liquid crystals is that, in general such compounds with large mesophase ranges are relatively viscous, while other compounds with low viscosity have small mesophase ranges or even only monotropic mesophases.
In the present invention, novel compounds have been found which permit a further improvement of the properties of liquid crystalline dielectrics.